Method and apparatus in a CDMA communication system

ABSTRACT

A novel and improved method and apparatus, in a WCDMA communication system ( 100 ), for informing a mobile station ( 101 ) of a downlink data frame time offset by determining the downlink data frame time offset, and transmitting the downlink data frame time offset via an Active Set Update message transmitted from a cell ( 102  or  103 ) to mobile station ( 101 ). Once an Active Set Update message is received, the data frame time offset information is provided to a timing block ( 201 ) which may determine data frame boundary of each downlink signal. Timing block ( 201 ) may then adjust the PN sequence timing corresponding to the downlink signal associated with the data frame time offset such that corresponding data symbols in each data frame are correctly soft combined in a combiner ( 210 ).

CLAIM OF PRIORITY UNDER 35 U.S.C. §120

The present Application for Patent is a divisional and claims priorityto patent application Ser. No. 09/576,063 entitled “Method and apparatusin a CDMA communication system” filed May 22, 2000, pending, andassigned to the assignee hereof and hereby expressly incorporated byreference herein.

BACKGROUND OF THE INVENTION

I. Field of the Invention

The present invention relates to communication systems. Moreparticularly, the present invention relates to a novel and improvedmethod and apparatus for soft handoff operations in a communicationsystem operating according to Wideband Code Division Multiple Access(WCDMA) principles.

II. Description of the Related Art

The 3rd Generation Partnership Project (3GPP) recently issued a versionof its technical specifications commonly referred to as the WCDMAstandard. The publication is well known in the art, and describes theoperational standard in a WCDMA system. The standard is still evolvingat the time of this application for a patent. A copy of the standard maybe obtained by contacting 3GPP, 650 Route des Lucoles—Sophia Antipolis,Valbonne—France, or by accessing the World Wide Web at the internetaddress www.3gpp.org. The latest version of the standard at the time ofthis application for a patent, and all previously revised publicationsof the same, are incorporated by reference herein.

Soft handoff operation is well known. A patent issued to Blakeney, II etal, with U.S. Pat. No. 5,640,414, and a patent issued to Gilhousen etal, with U.S. Pat. No. 5,101,501, both assigned to the assignee of thepresent application for a patent, both incorporated by reference herein,provide methods and systems for providing soft handoff in communicationsin a CDMA communication system. Generally, a mobile station in softhandoff operation receives and combines multiple signals transmittedfrom different cells in order to improve the overall decoding anddemodulation of the information. The signals transmitted from the cellsinvolved in the soft handoff operation carry the same information. Themobile station after decoding and demodulating the signals combines thedata metrics in a soft summing operation to add the data energy. As aresult, the information data is received with less error.

In CDMA systems, including WCDMA systems, each cell may have a uniquepseudo random noise (PN) sequence. Downlink signals transmitted fromeach cell are multiplied with the PN sequence as a part of the signalspreading function. The downlink signal received by a mobile station issubjected to a de-spreading operation in accordance with the PN sequenceused by the transmitter. The result of the de-spreading operations atthe data symbol level is combined with the result of de-spreadingoperation of other signals transmitted by other cells involved in thesoft handoff operation. Signals originating from the same cell butarriving at the mobile with different delays due to multipath can becombined in the same manner. A commonly known Rake receiver may be usedfor the combining operation.

When performing the soft-combining operation, it is important that themobile combines the matching symbols received from the cells involved insoft handoff. For example, the first symbol of a particular frame from afirst cell needs to be combined with the first symbol of the matchingframe from a second cell, and so on. The mobile station needs to knowthe frame timing of the data signals from each cell. Due to factorsincluding the possibility of cell timing being asynchronous with eachother, the possible timing offset between the PN spreading code frametiming and the data frame timing, and the uncertainties in thepropagation delay, the mobile cannot necessarily rely on the arrivaltiming of the symbols from each cell to determine which symbols shouldbe combined.

Therefore, there is a need for a method and apparatus for ensuring thata mobile station is aware of the data frame time offset of each downlinksignal transmitted by each cell involved in a soft handoff operation atthe mobile station.

SUMMARY OF THE INVENTION

The present invention is directed to a novel and improved method andapparatus, in a communication system, for informing a mobile station ofa downlink data frame time offset by determining the downlink data frametime offset, and transmitting the downlink data frame time offset from acell to the mobile station. The method and the accompanying apparatusfurther advantageously include receiving time offset informationmeasured by the mobile station. Determining the downlink data frame timeoffset is based on the time offset measured by the mobile station. Themobile can determine the timing of the frame boundaries based on theoffset information in the message. The timing offset measurement reportsent by the mobile and received by the network is used by a new cell tocome up with the frame offset. Once it comes up with the offset, itsignals the chosen offset to the mobile. The method and the accompanyingapparatus further advantageously include receiving the time offset via ameasurement report message. The time offset is measured by the mobilestation. The measurement report message is transmitted by the mobilestation to at least a base station controller controlling the cell. Oncethe mobile station receives an Active Set Update message, the data frametime offset information is provided to a timing block. Upon knowing dataframe time offsets associated with each downlink signal, the timingblock may determine data frame boundaries of each downlink signal. Thetiming block may then adjust the PN sequence timing corresponding to thedownlink signal associated with the data frame time offset such thatcorresponding data symbols in each data frame are correctly softcombined in a combiner. The result of the soft combining operation issubjected advantageously to a demodulator/decoder for retrieving theinformation.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, objects, and advantages of the present invention willbecome more apparent from the detailed description set forth below whentaken in conjunction with the drawings in which like referencecharacters identify correspondingly throughout and wherein:

FIG. 1 is a block diagram of downlink signal timing with respect to eachother and an uplink signal communicated between a mobile station and twocells in a soft handoff condition in a communication system; and

FIG. 2 is a block diagram of a portion of a receiver in a mobile stationfor determining time boundaries of data symbols based on data frame timeoffsets communicated to the mobile station via an Active Set Updatemessage in accordance with various embodiments of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A method and apparatus has been described in a patent applicationtitled: “Method and Apparatus for Fast WCDMA Acquisition”, with Ser. No.09/345,283, commonly assigned to the assignee of the presentapplication, and incorporated by reference herein. The Ser. No.09/345,283 application provides a description for a method and apparatusfor synchronization of a mobile station with a cell. Based on thesynchronization data, the mobile station is able to determine a dataframe time offset referenced to the mobile station uplink data frametiming. The uplink signal according to the standard is transmittedwithin a fixed time offset after reception of a downlink signal from acell. The uplink signal time offset may be fixed to 1024 chips. Themobile station, after determining the observed time difference,transmits the information to the cell via a message commonly known as anSFN-SFN message. The 3G TS 25.331 v3.2.0 (2000-03) document, section10.3.7.90, provides the description and the requirements for theobserved time difference message. The observed time differenceinformation is included in a cell measurement results message 10.3.7.3.The mobile station may periodically report to the cell. The mobilestation measures and reports the time difference in a unit of a chiptime.

The cell in connection with its base station controller sets data frametime offset for a downlink physical channel intended for the mobilestation that reported the SFN number on a measurement report message.The 3G TS 25.221 v3.2.0 (2000-03) document, section 7 and itssubsections, provides the description for the timing difference betweendifferent physical channels. For example, the primary common controlphysical channel is used as a timing difference for all physicalchannels, directly for the downlink and indirectly for the uplink. Thededicated physical channels intended for communication of data and voiceinformation are set in a data frame time offset from common control andpilot physical channels. The data frame time offset is set in a multipleof 256 chips from the common control and pilot physical channels.

The mobile station transmits a report message based on the measurementof the signals, such as the sync and the common pilot signals, for eachcell within a range. Each report message contains the time delaydifference measurement between the uplink timing and the downlink PNsequence frame timing from each cell (i.e. common and pilot channelframe timing) and an SFN number. The propagation distance from each basestation to the mobile station may be different. The observed timedifference measured by the mobile station for each cell may bedifferent. The downlink data frame time offset is set by each cell basedon its corresponding measurement report. Each cell may be operating onits own timing synchronously or asynchronously.

Referring to FIG. 1, in an exemplary situation, a mobile station 101 ina soft handoff situation with cells 102 and 103 receives two downlinksignals 111 and 112. A mobile station may be in soft handoff with morethan two cells; however, for simplicity of explanation, only a two-wayhandoff situation is described. Mobile station 101 transmits uplinksignal 113 to be received by cells 102 and 103. Each cell uses the timeoffset value reported in the measurement report message to set the dataframe time offset such that the downlink signal received by the mobilestation is approximately 1024 chips away in time from the uplink signaltransmitted from the mobile station. For illustrative purposes, forexample, uplink signal 113 is transmitted at time 120. Cell 102communicates to mobile station 101 via downlink 111. If the commoncontrol channel time reference of cell 102 is at time 121, cell 102 setsa data frame time offset 122 from time 121 such that after taking intoaccount the propagation delay 123, downlink signal 111 arrivesapproximately 1024 chips time away from uplink transmit time 120.

Mobile station 101 transmits a measurement report message to the networkincluding a base station controller 190 which informs cell 103 thetiming information necessary for determining a time frame offset 124associated with downlink 112. Cell 103 sets data frame time offset 124such that after including a propagation delay 125 associated withpropagation delay of downlink signal 112, downlink signal 112 arrives atmobile station 101 approximately 1024 chips time away from uplinktransmit time 120. The downlink signals 111 and 112, thus, arrive withinapproximately 1024 chips time of the uplink signal 113 at mobile station101. When mobile station 101 is in soft handoff with base stations 102and 103 via downlink signals 111 and 112, respectively, the softcombining operation may be performed.

In one embodiment of the invention, the data frame time offset 122 and124 are in increments of 256 chips time. The beginning of the PNsequence is set to coincide at the beginning of the cell common controland pilot channel time frame. Each downlink physical channel is spreadwith a PN sequence. As such, each downlink signal is at least 256 chipsin time offset from the other. Downlink signals to different mobiles canhave the same timing offset.

In one embodiment of the invention, when the mobile station 101 isperforming soft combining at the data symbol level, the data symbolsextracted from each downlink signal may be identified in terms oflocation in the data frame for an effective soft combining operationwith another data symbol in the same location of another data frame.Each data symbol may be between 4 and 512 chips in duration. The dataframe time offset is in increments of 256 chips. Data symbolstransmitted via the downlink signals then need to be identified at themobile station for the soft combing operation. Data frame time offset122 is set by cell 102 and data frame time offset 124 is set by cell103, depending on the measurements reported by the mobile station. Thedata symbols S1X received via downlink signal 111 need to be combinedwith data symbols S2X received via downlink signal 111 if mobile station101 is not aware of the data symbol boundaries, data symbols ofdifferent downlink signals may not correspondingly be combined. Sincethe cells attempt to adjust the transmit timing of the data channel sothat signals from different cells arrive at the mobile station atroughly the same time, one thing the mobile may do is combine symbolsthat are “closest” together in time. However, the possibility ofpropagation time relationships changing between the time of measurementreport and the start of the soft combining operation leaves ambiguitiesfor the mobile station for soft combining operation.

According to various embodiments of the invention, in a communicationsystem 100, a method and accompanying apparatus provide for informingmobile station 101 of a downlink data frame time offset by determiningthe downlink data frame time offset, and transmitting the downlink dataframe time offset via an Active Set Update message transmitted from acell to mobile station 101. The method and the accompanying apparatusfurther include receiving time offset information measured by mobilestation 101. Determining the downlink data frame time offset is thenbased on the time offset measured by mobile station 101. The method andthe accompanying apparatus further include receiving the time offset viaa measurement report message. The time offset is measured by mobilestation 101. The measurement report message is transmitted by mobilestation 101 to at least base station controller 190.

In one embodiment of the invention, mobile station 101 may adjust itsinternal timing after identifying the boundaries of data symbols for thecombining operation. An effective combining operation is possible sinceaccording to an embodiment of the invention, mobile station 101 is awareof the data frame time offset used by the cells. Mobile station 101 isable to determine data symbol boundaries since the downlink signal isspread by a PN sequence whose beginning coincides with the beginning ofthe common control channel and the pilot channel. Mobile station 101while in communication with a cell knows the timing boundary of thecommon control channel and the pilot channel, as deduced from timingassociated with the beginning of the PN sequence through an ongoingsynchronization process. The mobile station 101 measures the PN sequencetiming. Knowing the relationship between the PN sequence timing and thecontrol/pilot channel frame timing allows mobile station 101 to deducethe frame timing. Mobile station 101 de-spreads the downlink signal byremoving the PN sequence from the signal. The boundary of the first datasymbol is in time offset in increments of the data frame time offsetfrom the beginning of the PN sequence. Therefore, once the mobilestation knows the data frame time offset, it can determine the locationof the data frame boundary with respect to the PN sequence timing. Sincedata symbols are fixed in duration, boundaries of subsequent datasymbols after the first data symbol can easily be determined. The datasymbol combining operation takes place after the despreading operation.The mobile station can adjust the data symbol boundaries of a Rakereceiver finger to coincide to the data symbol boundaries outputtingfrom other fingers assigned to other downlink signals.

The 3G TS 25.331 v3.2.0 (2000-03) document, section 8.3.4, provides aprocedure for generating Active Set Update message in a soft handoveroperation, otherwise known as a soft handoff operation. The purpose ofthe Active Set Update is to update the active set of the connectionsbetween the mobile station and various cells involved in the softhandoff operation. The base station controller (BSC) 190 initiates theprocedure to modify the downlink connections. For all the signalingactivities including—ACTIVE SET UPDATE, MEASUREMENT REPORT, etc, themobile station is in communication with the BSC 190 (or radio networkcontroller (RNC) in WCDMA terminology). Each cell includes a transceiverthat does some physical layer processing (such as coding/decoding,interleaving/deinterleaving, and spreading/despreading.) The 3G TS25.331 v3.2.0 (2000-03) document, section 8.5.7, provides thedescription for generic actions for receipt of the information. A newSection 8.5.7.6.13 may be added that may describe according to variousembodiments of the invention what mobile station 101 is expected to dowhen it receives data frame time offset. Pursuant to the section10.3.6.15, Downlink DPCH information for radio link may be modified toinclude the data frame time offset associated with the downlink. Thedata frame time offset may be communicated in increments of a chip timeor 256 chip times.

Referring to FIG. 2, a simplified block diagram of a receiver in amobile station is shown for soft combining operation according tovarious embodiments of the invention. Downlink signals 204 and 205,after proper down conversion and signal processing (not shown), areinput to de-spreaders 202 and 203. Each downlink signal may beassociated with a cell. Once an Active Set Update message is received,the data frame time offset information is provided to a timing block201. Upon knowing the data frame time offset associated with eachdownlink signal, timing block 201 may determine the data frameboundaries of each downlink signal. Timing block 201 may then adjust thePN sequence timing corresponding to the downlink signal associated withthe data frame time offset such that corresponding data symbols in eachdata frame are correctly soft combined in a combiner 210. The result ofthe soft combined operation is provided to the demodulator/decoder 220for retrieving the information.

Accordingly, a soft combining operation at the data symbol level isperformed with high efficiency and accurate results.

The previous description of the preferred embodiments is provided toenable any person skilled in the art to make or use the presentinvention. The various modifications to these embodiments will bereadily apparent to those skilled in the art, and the generic principlesdefined herein may be applied to other embodiments without the use ofthe inventive faculty. Thus, the present invention is not intended to belimited to the embodiments shown herein but is to be accorded the widestscope consistent with the principles and novel features disclosedherein.

1. A method for a communication system, comprising: measuring downlinktime offset experienced at a mobile station between downlinktransmissions from base stations; communicating said measured timeoffset from said mobile station to at least one of said base stations;determining a downlink data frame time offset based on said measuredtime offset, wherein said downlink data frame time offset is in amultiple of predetermined number of chips; communicating said downlinkdata frame time offset information to said mobile station.
 2. The methodas recited in claim 1 wherein said communicating said downlink dataframe time offset information is by way of communicating an Active SetUpdate message.
 3. The method as recited in claim 1 wherein saidcommunicating said measured time offset is by way of communicating ameasurement report message from said mobile station.
 4. The method asrecited in claim 1 further comprising: adjusting timing of a time offsetadjuster in said mobile station for adjusting data symbol timingaccording to said downlink data frame time offset information and foridentifying corresponding data symbols for a soft combining operation.5. The method as recited in claim 4 further comprising soft combiningsaid corresponding data symbols.
 6. An apparatus for a communicationsystem, comprising: a receiver for measuring downlink time offsetexperienced at a mobile station between downlink transmissions from basestations; a first transmitter for communicating said measured timeoffset from said mobile station to at least one of said base stations; acontroller for determining a downlink data frame time offset based onsaid measured time offset, wherein said downlink data frame time offsetis in a multiple of predetermined number of chips, and a secondtransmitter for communicating said downlink data frame time offsetinformation to said mobile station.
 7. The apparatus as recited in claim6 wherein said second transmitter is further for communicating saiddownlink data frame time offset information by way of communicating anActive Set Update message.
 8. The apparatus as recited in claim 6wherein said first transmitter is further for communicating saidmeasured time offset by way of communicating a measurement reportmessage from said mobile station.
 9. The apparatus as recited in claim 6wherein said receiver is further for adjusting timing of a time offsetadjuster in said mobile station for adjusting data symbol timingaccording to said downlink data frame time offset information and foridentifying corresponding data symbols for a soft combining operation.10. The apparatus as recited in claim 9 wherein said receiver is furtherfor soft combining said corresponding data symbols.
 11. A mobile stationfor a communication system, the mobile station comprising: a receiverconfigured to measure a downlink time offset experienced at the mobilestation between downlink transmissions from at least two base stations;a transmitter for communicating the measured downlink time offset to atleast one of the at least two base stations; said receiver furtherconfigured for receiving a downlink data frame time offset informationdetermined based on the communicated measured downlink time offset. 12.The mobile station of claim 11, further comprising: a time offsetadjuster for adjusting received data symbol timing according to thedownlink data frame time offset information and for identifyingcorresponding data symbols for a soft combining operation.
 13. A methodfor a mobile station in a communication system, the method comprising:measuring a downlink time offset experienced at the mobile stationbetween downlink transmissions from at least two base stations;communicating the measured downlink time offset to at least one of theat least two base stations; receiving a downlink data frame time offsetinformation determined based on the communicated measured downlink timeoffset.
 14. The mobile station of claim 13, further comprising:adjusting received data symbol timing according to the downlink dataframe time offset information and for identifying corresponding datasymbols for a soft combining operation.
 15. A method for a communicationsystem, comprising: determining a downlink data frame time offset basedon a measured time offset, wherein said measured time offset is a timeoffset experienced at a mobile station between downlink transmissionsfrom base stations, wherein said downlink data frame time offset is in amultiple of predetermined number of chips; communicating said downlinkdata frame time offset information to said mobile station.
 16. Themethod as recited in claim 15 wherein said communicating said downlinkdata frame time offset information is by way of communicating an ActiveSet Update message.
 17. The method as recited in claim 15 wherein saidmeasured time offset is received by way of receiving a measurementreport message from said mobile station.
 18. The method as recited inclaim 15 wherein said communicating said downlink data frame time offsetinformation to said mobile station is for adjusting timing of a timeoffset adjuster in said mobile station for adjusting data symbol timingaccording to said downlink data frame time offset information and foridentifying corresponding data symbols for a soft combining operation.19. The method as recited in claim 18 wherein said communicating saiddownlink data frame time offset information to said mobile station isfurther for soft combining said corresponding data symbols.
 20. Anapparatus for a communication system, comprising: a controller fordetermining a downlink data frame time offset based on a measured timeoffset, wherein said measured time offset is a time offset experiencedat a mobile station between downlink transmissions from base stations,wherein said downlink data frame time offset is in a multiple ofpredetermined number of chips; a transmitter for communicating saiddownlink data frame time offset information to said mobile station. 21.The apparatus as recited in claim 20 wherein said transmitter is furtherfor communicating said downlink data frame time offset information byway of communicating an Active Set Update message.
 22. The apparatus asrecited in claim 20 further comprising a receiver for receiving saidmeasured time offset by way of receiving a measurement report messagefrom said mobile station.
 23. The apparatus as recited in claim 20wherein said transmitter is further for communicating said downlink dataframe time offset information to said mobile station for adjustingtiming of a time offset adjuster in said mobile station for adjustingdata symbol timing according to said downlink data frame time offsetinformation and for identifying corresponding data symbols for a softcombining operation.
 24. The apparatus as recited in claim 23 whereinsaid transmitter is further for communicating said downlink data frametime offset information to said mobile station for soft combining saidcorresponding data symbols.